1. Field of the Invention
The invention relates to methods for producing compositions comprising organosilicon compounds and hydrophobic silicas and to the use thereof as defoamers.
2. Description of the Related Art
In many liquid systems, more particularly in aqueous systems, which include surface-active compounds as desired or else as unwanted constituents, it is possible for problems to occur as a result of foaming if these systems come into more or less intensive contact with gaseous substances, such as during the gassing of wastewaters, during the intensive stirring of liquids, during distillation, washing or coloring operations, or during dispensing procedures, for example.
This foam can be controlled by mechanical means or through the addition of defoamers. Siloxane-based defoamers have proven particularly appropriate. Defoamers based on siloxanes are prepared according to U.S. Pat. No. 3,383,327 A, for example, by heating hydrophilic silica in polydimethylsiloxanes. Using basic catalysts allows the effectiveness of such defoamers to be improved, as disclosed in U.S. Pat. No. 3,560,401 A. An alternative is to disperse hydrophobized silica in a polydimethylsiloxane, in accordance, for example, with DE 29 25 722 A1.
Nevertheless, the effectiveness of the resulting defoamers is usually in need of improvement. Thus U.S. Pat. No. 4,145,308, for example, describes a defoamer preparation which as well as a polydiorganosiloxane and silica also comprises a copolymer made up of (CH3)3SiO1/2 and SiO2 units.
Copolymers made up of (CH3)3SiO1/2 and SiO2 units are said to be advantageous also in combination with siloxanes which carry terminal long alkyl groups, as described in EP-A 301 531 (corresponding U.S. Pat. No. 4,919,843 A). In these formulations, mixtures of fumed and precipitated silicas are used as well.
EP-B 726 086 achieves enhanced effectiveness for the defoamer formulation by using a mixture of pretreated hydrophobized silica and silica hydrophobized in situ. In this case, first the hydrophilic silica is mixed with the polysiloxane. This is followed by the hydrophobizing in situ at elevated temperatures, and, after cooling has taken place, the pretreated hydrophobized silica is added.
In strongly foaming, surfactant-rich systems, however, such as in laundry detergents or the black liquor produced in papermaking, for example, the known defoamer formulations do not always have a sufficiently long-lasting effectiveness and compatibility.